Individual differences in tobacco use (or, smoking initiation, heritability, h2=75%), cannabis use (h2=48%), nicotine dependence (h2=60%) and cannabis abuse/dependence (h2=50-60%) are partly attributable to genetic factors (or heritability, h2) (27;28). There is robust evidence from twin studies suggesting that these heritable influences on tobacco and cannabis use and misuse overlap with each other, with liability to use and misuse of other psychoactive substances and with a general vulnerability to externalizing behaviors.
